This definitely feels like an HR thing to me (whether there's an HR department, or handled by a manager). There's nothing with masturbation, but the way it seems to always be happening, he must *know* that he's likely to overheard or otherwise noticed, and *know* that that's likely to make other people uncomfortable, and *know* that many people would feel threatened by forcing awareness of his sexual activities on them, and keep on doing it anyway. Often enough that it's not just once, it's a regular problem.

That said, if LW doesn't want the embarrassment of bringing it up with HR, I imagine knocking on the stall door and asking him to stop is likely to be extremely effective. That guy's the one who broke the secret veil of not acknowledging sex stuff or private cubical stuff, not the LW.